An optical parametric amplifier typically comprises a material that has a nonlinear, i.e., amplitude-dependent, response to each incident light wave. In addition to an information-modulated wave to be amplified, a pumping wave of another frequency is applied to the material to interact with and transfer energy to the information-modulated wave. The amplification of the information-modulated wave produced by this transfer of energy is called parametric gain.
The main line of development both of optical second harmonic generators and of optical parametric amplifiers has centered around the use of birefringent materials to obtain phase matching, which enables traveling wave parametric amplification. Phase matching is the process of making the sum of the signal and idler wave vectors equal to the pumping wave vector. A wave vector is related to the product of index of refraction and frequency, or to index divided by wavelength.
In general, classical phase matching (e.g., via angle or thermal tuning) requires a certain combination of intrinsic birefringence and dispersion of refractive indices. New small molecular weight crystalline organic nonlinear optical materials with high second harmonic susceptibility have been reported in literature such as ACS Symposium, Series No. 233, pages 1-26, 1983 by Garito et al. These organic materials usually possess high intrinsic birefingence and positive dispersion so that phase matching can be achieved with a single crystal. Even if phase matching can be achieved with the new types of organic materials having high nonlinear optical susceptibility, the low beam power of a diode laser significantly limits the power conversion efficiency. The high birefringence of the organic materials also lowers the conversion efficiency because of beam walk-off.
An alternative means to achieve phase matched conditions is the use of dispersion properties for different modes in a waveguide. Since the energy is confined laterally to a narrowly constricted space, a high light intensity is possible with a relatively low power source. If the waveguide geometry and refractive indices of the guiding region and cladding region are such that: EQU .beta.=.beta..sub.n (.omega..sub.3)-.beta..sub.m (.omega..sub.2)-.beta..sub.1 (.omega..sub.1)=0 (1)
then the phase matching condition is established. Here, .beta..sub.i is the propagation constant of the i-th mode. The conversion efficiency is generally quadratically dependent on the overlap integral between the modes: EQU F=.intg.E.sub.n (.omega..sub.3, z)E.sub.m (.omega..sub.2, z)E.sub.1 (.omega..sub.1, z)dz
where E.sub.k is the normalized electric field of the k-th mode across the waveguide. In general, the overlap between the waveguide modes is limited, and the value of the overlap integral is also small. This approach has been utilized for second harmonic generation phase matching in waveguides derived from organic materials, as reported in Optics Commun., 47, 347 (1983) by Hewig et al. However, the level of second harmonic conversion efficiency is low, suggesting no practical parametric amplification application.
Of background interest with respect to the present invention are U.S. Pat. Nos. 3,267,385; 3,660,673; and 3,831,038 which describe optical parametric amplifier devices with inorganic nonlinear optical waveguiding means. Also of interest is literature relating to spatially periodic nonlinear structures for modulation of electromagnetic energy. The pertinent literature includes IEEE J. of Quantum Elect., QE-9 (No. 1), 9 (1973) by Tang et al; Appl. Phys. Lett., 26, 375 (1975) by Levine et al; Appl. Phys. Lett., 37(7), 607 (1980) by Feng et al; and U.S. Pat. Nos. 3,384,433; 3,407,309; 3,688,124; 3,842,289; 3,935,472; and 4,054,362.
The thin film waveguides with a periodically modulated nonlinear optical coefficient as described in the literature are either inorganic optical substrates with material fabrication disadvantages, or they are organic materials which are in the liquid phase, such as a liquid crystal medium or a thin film of nitrobenzene which require a continuously applied external DC electric field.
Of particular interest with respect to the present invention is literature relating to the dispersive properties of a thin film optical waveguide for TE and TM modes, as expressed in analytic terms defining the variation of the effective refractive index with respect to one or more physical parameters in the waveguiding medium. The pertinent literature includes J. Appl. Phys., 49(9), 4945 (1978) by Uesugi et al; Appl. Phys. Lett., 36(3), 178 (1980) by Uesugi; Nonlinear Optics: Proceedings Of The International School Of Materials Science And Technology, Erice, Sicily, July 1-14, 1985 (Springer-Verlag), pages 31-65 by Stegeman et al; Integrated Optics, Volume 48, pages 146-151 by Ostrowsky (Springer-Verlag, 1985); Integrated Optics, Volume 48, pages 196-201 by Bava et al (Springer-Verlag, 1985); and Appl. Opt., 25(12), 1977 (1986) by Hewak et al.
There is continuing interest in the development of compact and efficient nonlinear optical devices, such as parametric oscillators and amplifiers.
Accordingly, it is an object of this invention to provide an optical parametric amplifier device with an organic nonlinear optical waveguiding medium which is adapted to amplify the intensity of an input signal beam.
It is another object of this invention to provide an optical parametric amplifier device with a polymeric nonlinear optical waveguide channel having a spatial periodic structure for quasi-phase matching of propagating wave vectors, and with a refractive index tuning means for efficient phase matching.
Other objects and advantages of the present invention shall become apparent from the accompanying description and drawings.